This invention relates generally to fiber optical phase modulation techniques, and more particularly, to phase modulation in a discrete or digital sense, as in the modulation of the phase of a carrier signal with a sequence of digital signals. One common method of phase modulation is quadrature phase-shift keying (QPSK), in which digital signals are encoded as abrupt phase shifts in a radio-frequency carrier signal.
There are some applications involving QPSK, and other similar forms of phase modulation, in which the use of a conventional microwave transmitting medium is inadvisable or impracticable. One example is a phased array antenna used to provide inertialess scanning of an antenna beam. The principle of the phased array antenna, in a transmitting mode, is that each element of an antenna array is provided with a signal of which the phase is separately controlled. Control of the phase angle of each transmitting element allows the composite beam from the array to be angularly steered. In theory, the beam can be steered over an entire hemispherical region, but in practice arrays are employed to cover somewhat smaller segments of angular space.
One of the principal difficulties of phased array antenna systems is that each antenna element requires its own microwave feed line, which is typically a waveguide or coaxial cable, and its own phase shifter to control the phase of the signal applied to the antenna element. In many applications, the antenna has to be mounted for mechanical movement, giving rise to repeated flexing of the cables and waveguides. Another significant factor is that, for large antenna arrays, the weight of a hundred or more antenna feed cables can pose serious practical problems. In addition, signals transmitted in coaxial cables are prone to electromagnetic interference. Although it is generally appreciated that the use of optical fibers would obviate many of these problems, prior to this invention there has been no availble technique for phase- shifting an optical beam in a discrete or digital manner, either for purposes of data modulation or for antenna beam steering.
It will be appreciated from the foregoing that there is a need for a simplified technique for transmitting signals to or from phased array antennas, as well as a simplified approach for phase modulation of an optical signal. The present invention fulfills this need.